Machines such as vehicles or ships, and in particular off-highway machinery such as wheel loaders, excavators, dozers, tractors, harvesters and similar heavy duty machines often operate with hydraulic steering systems. In fluid controllers for steering a vehicle or a ship, the valve member, e.g. in the form of a spool, often forms a neutral configuration and two operating configurations on opposite sides of neutral, corresponding to a left-turn and a right turn of the vehicle. Similar fluid controllers are, however, also applied in machines of a completely different kind, e.g. for controlling lifting in a crane, movement of robots, or in general for controlling a machine.
Typically, the steering system receives a steering input from an operator of the machine who thereby indicates a desired activity of the machine, e.g. a desired angular movement of a steering wheel of a vehicle. The steering input could be provided by an operator via a handle such as a steering wheel, a joystick, a push button etc. In a fluid controller, the steering input is converted into a hydraulic flow to a pressure operated device, e.g. a hydraulic actuator or cylinder which moves the steering wheels of a vehicle. A fluid controller of the above mentioned kind is disclosed e.g. in U.S. Pat. No. 5,992,458 and in U.S. Pat. No. 5,638,864 wherein the controller is disclosed in combination with an unequal area steering cylinder. Typically, the fluid controllers comprise a housing with a valve member which is movable in the housing and which cooperates with the housing to open and close various passages between the source and ports of the pressure operated device and possibly between other ports of the pressure operated device and a receptacle for the fluid.
Typically, the steering input which is provided on the handle is converted into a reference which is indicative of a desired position of the valve member relative to the housing. Based on the reference, the valve member is moved in the housing, e.g. by use of an electrically or hydraulically operated actuator. Some of the existing fluid controllers comprise a sensor which provides an output indicative of an actual position of the valve member relative to the housing.
To detect faults in the system, an integrated observer may determine a theoretical model output which is indicative of a theoretically correct position of the valve member relative to the housing based on a specific reference. The observer may thereby provide a residual value which indicates a possible difference between the output as provided by the sensor and the model output determined from the reference and a model of the system.
By comparing the residual value with a threshold value, an alert can be provided when a specific reference does not lead to a desired position of the valve member relative to the housing. This situation may occur e.g. if movement of the valve member in the housing is restricted.
U.S. Pat. No. 6,766,230, U.S. Pat. No. 6,470,300 and U.S. Pat. No. 6,687,585 all disclose various fault detection systems and methods for vehicles, engines etc.
In the existing fluid controllers, a discrepancy exist between on the one hand to set the threshold value relatively close to zero thereby to detect a majority of the situations in which the valve member does not move in an intended way, and on the other hand, not to be alert unless there is an error which is significant.